Which of the following is a mechanism of placental transport?

The correct answer highlights the process of passive diffusion as a mechanism of placental transport. Passive diffusion is a fundamental process through which substances move across the placental barrier from an area of higher concentration to an area of lower concentration without the need for energy expenditure. This mechanism is essential for the delivery of nutrients and oxygen from maternal to fetal blood.

In the context of placental transport, passive diffusion plays a vital role, particularly for small, non-polar molecules, or gases like oxygen and carbon dioxide, which readily diffuse through the placental membranes due to concentration gradients. This allows for efficient gas exchange, crucial for fetal development.

Active transport requires energy to move substances against a concentration gradient, which is also important in the placenta but is not the primary mechanism being referenced in this question. Osmosis refers specifically to the movement of water across a semipermeable membrane, and while it does occur in the placenta, it is not the primary method for transporting nutrients. Diffusion through the skin does not apply to placental transport mechanisms and is more relevant to transdermal absorption in other contexts. Understanding the primary role of passive diffusion in the placenta can help one grasp how essential nutrients and gases are efficiently exchanged between the mother and fetus.